(a) Field of the Invention
This invention generally relates t the art of polymer molding and specifically to a method of producing a panel having an essentially integral or "monolithic" coating of a cross-linked polymer matrix formed in a closed mold containing a panel core.
(b) Prior Art of the Invention
Coating methods can be divided roughly into two groups depending upon whether a preformed coating layer is applied to a substrate, e.g. as in laminating techniques where a thermoset layer, frequently on a thin support layer, is adhesively connected with a structural support such as a chipboard or the like material in a platen press, or whether the coating is formed upon the structural support, e.g. by applying a liquid (including viscous) composition onto the support and by drying, setting or curing the liquid composition.
The advantage of forming the coating on the substrate is that "monolithic" coatings, i.e. completely encompassing or "enrobing" the support, can be obtained but production of "heavy" coats, e.g. in the thickness range of typically between about 1 mm and about 5 mm or more, is not normally possible by typical coating methods, such as by dipping or spraying.
The advantage of applying preformed coats is that heavy coats can be applied at least as easily as thin coats but the coating cannot normally be made monolithic, nor adapted to shaped surface structures (i.e. substantially deviating from planarity).
Further, the nature of the polymer or polymers that form the continuous phase of the coat will have a substantial impact upon the available spectrum of techniques; while thermoplastic materials provide for most variation as regards application technique, a requirement of maximum resistance against humidity, chemical or abrasive attack, etc. requires coats or coatings of cross-linked or "duromeric" polymers, such as epoxy or cross-linked polyesters, polyurethanes, etc., generally with a high portion of inorganic filler dispersed in the cross-linked polymer phase or matrix.
For example, table tops, front panels and many other more or less typical furniture components for heavy duty use in laboratories, hospitals, kitchens, swimming pools, bathrooms, etc. having maximum resistance against humidity, abrasion, intensive cleaning, disinfectants, organic solvents, heat, etc. combined with pleasing external appearance and touch require heavy integral coatings of the cross-linked or duromeric type which can be obtained but by casting methods, such as disclosed in U.S. Pat. No. 3,832,264 (Barnette) or in Assignee's Swiss Patent No. 557,232.
While the Barnette method produces what is called a cast plastic laminate in the patent and does not contemplate integral coating, the cast molding method illustrated in FIGS. 6 to 9 of Swiss Patent No. 557,232 does yield panels with heavy monolithic coats optimally suited for heavy duty uses of the type mentioned above.
To this end, the Swiss Patent teaches using a cast mold formed by a lower tray portion and a co-acting and generally flat upper lid portion; the panel core, e.g. a rectangular piece of chip-board, is attached with a few, say 2 to 6, mounting pins on the lid but at a distance therefrom; then, the mold is closed by engaging the rim of the tray portion with a generally co-extensive inner surface area of the lid portion, usually including sealing means, e.g. a rubber insert, provided at the interface.
The shape (including dimensions) of the mold cavity is selected so as to define, in essence, the external shape (and dimensions) of the final integrally coated panel while the shape (including dimensions) of the core and its arrangement on the lid are selected such that a free interspace which corresponds, in essence, with the coat that is to be cast and extends substantially around the entire core (excluding but a few small spots formed by the mounting pins) remains.
Mounting brackets, pneumatic clamps or the like means are used to hold the lid in sealing engagement with the tray and the closed mold is arranged in an inclined position, i.e. so as to include an angle of less than 90.degree. and more than 0.degree., and so that the inlet opening is at the lowest part of the closed mold while the outlet opening is at the most elevated point.
Injection is continued until the mold is filled; the outlet opening in the lid at the upper end of the closed mold when in casting position is provided to permit displacement of air and to show complete mold filling by emergence of liquid resin composition.
The closed mold is held at ambient conditions until the resin composition has set (solidified by gel formation) to an extent sufficient for removal of core plus coat from the mold (using conventional mold release means or agents). Final curing is effected at an elevated temperature depending upon the cross-linking requirements of the particular composition.
I have found, however, that even if core mounting, mold closure, resin injection, positioning of the mold and withdrawal of the coated product are automated and optimized, notably as regards positioning of the mold during injection, this conventional method leads to a relatively high rate of products having visible surface irregularities including undesired "textures", macroscopic inhomogeneities or physical defects, e.g. voids or pores.